The invention relates to an antenna structure which comprises two resonance frequency bands, i.e., which can be used as the antenna of a radio set in two frequency ranges.
Mobile phones operating in cellular networks are rapidly becoming the most important means of personal communications used to convey speech, telefax messages, and data in electric form via communications networks from one user to another. Such a mobile phone is used in the following as an example of a radio set in connection with which the antenna of the invention can be used.
Cellular telephone systems are used in different parts of the world, where operating frequency ranges deviate considerably from one another. Of digital cellular telephone systems, the operating frequencies of the GSM system (Groupe Speciale Mobile) are around 900 MHz, those of the JDC (Japanese Digital Cellular) in the ranges of 800 and 1500 MHz, those of the PCN (Personal Communication Network) in the range of 1800 MHz, and those of the PCS (Personal Communication System) in the range of 1900 MHz.
The mobile phones intended for these systems generally use simple cylindrical coil antennas, i.e., helical antennas, or rod antennas formed of straight conductors, due to their low manufacturing costs and relatively high electrical performance. The resonance frequency of the antenna is defined by its electric length, which has to form a certain part of the wavelength of the radio frequency in use. The electric length of a helical antenna used on mobile phone frequencies is preferably, e.g., 3.lambda./4, 5.lambda./4, or .lambda./4, in which .lambda. is the wavelength being used. Correspondingly, the electric length of a rod antenna is preferably, e.g., .lambda./2, 5.lambda./8, 3.lambda./8, or .lambda./4. Solutions are also known in which the rod part and the helix part can be connected alternately to the antenna port of the radio set, as well as rod-helix serial connections which can be pushed partly inside the telephone (e.g., patent publication WO 92/16980). The aim of these solutions is generally to make the antenna as small as possible when in storage and transportation position, but such that it can be pulled out when necessary for a better connection.
Since the resonance frequency of the antenna according to prior art depends on the wavelength in the manner described above, one antenna can only be used in a mobile phone intended for the cellular telephone system of one frequency range. However, in some cases it is preferable that one and the same telephone can also be used in another frequency range. In this case, a viable antenna solution is needed in addition to other appropriate RF-parts.
The easiest solution would be to provide the telephone with at least two separate antennas, of which the user could place in his telephone the one corresponding to the frequency range of the system he is using at that time. However, it is probable that the needed replacement antenna cannot be found at that time. Continuous replacing of antennas also strains the antenna plug and may cause contact disturbances in the course of time. Another alternative is to prepare at least two fixed, differently dimensioned antennas at different points of the telephone, of which, by using a switch, the user selects the one corresponding to the frequency range of the system being used. This increases the number of parts of the telephone and, consequently, the manufacturing costs.
U.S. Pat. No. 4,442,438 discloses an antenna structure which resonates on two frequencies and comprises essentially two helices HX1, HX2 and one rod element P1, according to FIG. 1. Helices HX1 and HX2 are installed sequentially in the direction of the symmetry axis of the structure and their adjacent ends A1 and A2 form the feed point of the composite structure. Rod element P1 is partly inside the upper helix HX1, extending slightly outside, and its feed point A3 is at the lower end thereof. RF signals are brought to this feed point A3 through coaxial conductor KX joining with the symmetry axis of the structure, the coaxial conductor going through the lower helix HX2 . Feed point A3 of the rod element is connected to lower end A1 of the upper helix and the lower helix is connected, at the upper end A2 thereof, to the conducting and grounded sheath of coaxial conductor KX. The first resonance frequency of the structure is the resonance frequency of the combined structure formed by helices HX1 and HX2; 827 MHz in the exemplifying embodiment. The second resonance frequency of the structure is the common resonance frequency of the upper helix HX1 and rod element P1, which is 850 MHz in the exemplifying embodiment. Helix HX1 and rod element P1 are thus dimensioned so that they comprise essentially the same resonance frequencies.
The structure disclosed in the U.S. patent is relatively complex and its physical length in the direction of the symmetry axis is the sum of the physical lengths of lower helix HX2 and rod element P1. The most troublesome point of the structure from the point of view of the manufacturing technique is the feed point arrangement in the middle of the antenna, in which lower end A3 of the rod element and lower end A1 of the upper helix have to be galvanically connected, and the lower helix has to be connected at its upper end A2 to the sheath of the coaxial conductor feeding the rod element. The difference between the two resonances obtained by using the structure is small according to the material disclosed in the patent because upper helix HX1 and rod element P1 have to be dimensioned so that they have essentially the same common resonance frequencies, therefore, it cannot be implemented in telephones operating on the GSM and PCN frequencies, for example. The descriptive part of the patent thus suggests, for the object of the invention, that the resonance frequency range of the mobile phone antenna should be widened so that it would better cover the whole frequency band in one cellular telephone system. It would be difficult to apply the structure to more than two resonance frequencies.
The object of the present invention is to provide an antenna suitable for radio communications, especially for mobile phones, comprising at least two discrete resonance frequency ranges. Another object of the invention is to disclose an antenna structure whose resonance frequencies can be freely selected when designing the antenna. A further object of the invention is to disclose a mobile phone antenna with at least two frequencies whose structure is simple and reliable and which is well-adapted to mass production. A further object of the invention is to disclose a small-size, at least dual-frequency mobile phone antenna.